It is important to remember that not all on-line communication is equal. For instance, I don't expect the same level of grammar and spelling in a chat room as I do in an on-line newspaper reporting on a feature story that has been in the works for weeks on end. In the former, one types on the fly, with speed being the essence, and without the expectation that the words typed will last any longer than the chat session (logs notwithstanding). In the latter, each word is expected to be weighted carefully as the novel is essentially forever. There is no real pressure to type something "right this instant", and there is a staff of proofreaders whose sole job is to eliminate errors such as typographical and grammatical.
The Free Republic forum is a lot closer to the latter than the former. The words typed here do last forever (unless yanked by the moderators for violating the posting guidelines), and there is relatively little pressure to type something "right this instant". However, there is no staff who looks for typographical errors (unless you happen to have someone else at the desk while you preview your post), and there is no spell-check or grammar-check built into the posting system.
What is truly key on the forums is having a coherent and well-supported point. Proper grammar and spelling do help a lot in making the point coherent, but the occassional slip usually doesn't ruin the point beyond the point of readability. Conversely, you could post something that is grammatically and typographically perfect, yet if it doesn't have a coherent, well-supported and persuasive point, you'll almost certainly be ridiculed for it.
Yes...we usually communicate in the vernacular of the chatroom! When friends "sit around the fire" to bat around some ideas, we don't usually feel the need to edit and do re-writes! That's just TOO DAMNED MUCH WORK!!!!